The present invention relates to a switching power supply formed of a DC/DC converter, and more specifically, to a switching power supply which supplies an output voltage of the DC/DC converter through wiring to a load located away from the DC/DC converter and controls the output voltage of the DC/DC converter at a constant value by remotely detecting a terminal voltage of the load.
This kind of a DC/DC converter according to the prior art is explained with reference to FIG. 4, which is a block diagram of the DC/DC converter according to the prior art.
In FIG. 4, a symbol SW designates a DC/DC converter; E is a direct voltage source; and L is a load located away from the DC/DC converter SW. The DC/DC converter SW comprises a switching portion 1 which switches on and off an electric current of the direct voltage source E by switching elements, such as transistors, thyristors, etc. and supplies the electric current switched on and off to a transformer 2; a rectifying and smoothing portion 3 which rectifies and smooths an output voltage of a secondary side of the transformer 2 electrically insulated from the direct voltage source E; output terminals T1, T2 from which an output voltage of the rectifying and smoothing portion 3 is supplied through distribution wires L1, L2 to the load L; remote detecting terminals T3, T4 from which a voltage between terminals of the load L is remotely detected through detecting leads L3, L4; a remote output voltage detecting portion (hereinafter referred to as "remote detecting portion") 4 formed of a voltage dividing circuit and so on, which detects a voltage applied between the remote detecting terminals T3, T4; an error amplifying portion 5 which amplifies a difference between an output voltage of the remote detecting portion 4 and a reference voltage from the first reference voltage generating portion 6; a comparing portion 7 which compares a voltage difference from the error amplifying portion 5 with a signal corresponding to ON-OFF duty ratio of the switching portion 1; an oscillating portion 8 which changes the ON-OFF duty ratio of the switching portion 1 based on an output from the comparing portion 7 and supplies the changed ON-OFF duty ratio to the switching portion 1; and an overvoltage protecting portion 9 which is connected to the positive voltage side of the rectifying and smoothing portion 3 through a resistor 10, and stops supply of the signal from the oscillating portion 8 to the switching portion 1 by detecting the overvoltage based on the output voltage of the rectifying and smoothing portion 3. The error amplifying portion 5, the comparing portion 7 and the oscillating portion 8 constitute a control portion for changing the ON-OFF duty ratio of the switching portion 1. A resistor R20 is connected between the negative output voltage side of the rectifying and smoothing portion 3 and the negative input voltage side of the remote detecting portion 4.
In the switching power supply of FIG. 4, as known already, the electric current of the direct voltage source E is switched on and off in the switching portion 1 and the electric current switched on and off is supplied to the primary side of the transformer 2. The alternating voltage generated on the secondary side of the transformer 2 is rectified and smoothed in the rectifying and smoothing portion 3, and the direct voltage is supplied to the load L from the rectifying and smoothing portion 3 via the output terminals T1, T2. The voltage between the terminals of the load L is applied to the remote detecting terminals T3, T4 through the detecting leads L3, L4 and is detected in the remote detecting portion 4. The voltage difference between the output voltage from the remote detecting portion 4 and the reference voltage from the first reference voltage generating portion 6 is supplied from the error amplifying portion 5 to the comparing portion 7, and the supplied voltage difference is compared in the comparing portion 7 with the signal corresponding to the ON-OFF duty ratio in the switching portion 1.
The comparing portion 7 controls, in response to the voltage difference supplied from the error amplifying portion 5, ON-OFF duty ratio of the pulse signal fed from the oscillating portion 8 to the switching portion 1. Based on the controlled ON-OFF duty ratio of the pulse signal, magnitude or amount of the electric power supplied from the switching portion 1 is changed, and the output voltage from the DC/DC converter SW is controlled at a predetermined constant value.
In the prior art shown in FIG. 4, in case the detecting leads L3 or L4 of the load L connected to the remote detecting terminal T3 or T4 of the DC/DC converter is disconnected, the disconnection of the detecting leads L3 or L4 can not be detected. When the detecting lead L3 or L4 is disconnected, the output voltage of the DC/DC converter SW is only supplied to the remote detecting portion 4 through the resistors R10, R20. In this case, since the output voltage of the DC/DC converter is higher than the voltage between the terminals of the load L for the amount of the voltage drop through the distribution wires L1, L2, the control portion operates to lower the ON-OFF duty ratio of the switching portion 1, so that the voltage practically supplied to the load L is lowered.
In view of the foregoing, an object of the present invention is to obviate the defects of the prior art and to provide a switching power supply which can detect disconnection of the distribution wires of the load connected to the remote detecting terminals.
Another object of the present invention is to provide a switching power supply which can keep the output voltage at a predetermined constant value suitable for the load even when the distribution wire of the load connected to the remote detecting terminal is disconnected.